The allergy-preventing effect of breast-feeding remains controversial, possibly because of individual variations in the composition of the breast milk. The aim of this study was to investigate the concentrations of cytokines involved in allergic reactions and IgA antibody production in breast milk from allergic and nonallergic mothers. The cytokine concentrations were determined in colostrum and 1-mo milk samples from 24 mothers with, and 25 mothers without, atopic symptoms, using commercial ELISA kits. The immunosuppressive cytokine transforming growth factor-&bgr; was predominant and was detectable in all milk samples. IL-6 was detected in the majority of colostral and mature milk samples, whereas the other cytokines were less commonly detected. The concentrations of IL-6, IL-10, and transforming growth factor-&bgr;, which are all involved in IgA synthesis, correlated with each other and with total IgA concentrations in colostrum. The concentrations of IL-4 were higher in colostrum from allergic than nonallergic mothers, and similar trends were seen for IL-5 and IL-13. In conclusion, transforming growth factor-&bgr; and IL-6 were the predominant cytokines in human milk. The correlation between the concentrations of cytokines involved in IgA synthesis,i.e.IL-10, IL-6, and transforming growth factor-&bgr;, may explain the stimulatory effect on IgA production in breast-fed babies. Varying concentrations of IL-4, IL-5, and IL-13 may explain some of the controversy regarding the possible allergy-preventive effect of breast-feeding.